Name

    ARB_query_buffer_object

Name Strings

    GL_ARB_query_buffer_object

Contact

    Daniel Rakos (daniel.rakos 'at' amd.com)

Contributors

    Daniel Rakos, AMD
    Graham Sellers, AMD
    Christophe Riccio, AMD

Notice

    Copyright (c) 2013 The Khronos Group Inc. Copyright terms at
        http://www.khronos.org/registry/speccopyright.html

Status

    Complete. Approved by the ARB on June 3, 2013.
    Ratified by the Khronos Board of Promoters on July 19, 2013.

Version

    Last Modified Date: 7 March 2016
    Author Revision: 3

Number

    ARB Extension #148

Dependencies

    OpenGL 1.5 is required.

    This extension is written against the OpenGL 4.3 (core) specification.

Overview

    Statistics about the operation of the OpenGL pipeline, such as the number
    of samples that passed the depth test, the elapsed time between two events
    or the number of vertices written by transform feedback can be retrieved
    from the GL through query objects. The result of a query object is
    acquired by the application through the OpenGL API into a client provided
    memory location. Should the result returned by the API be required for use
    in a shader, it must be passed back to the GL via a program uniform or
    some other mechanism. This requires a round-trip from the GPU to the CPU
    and back.

    This extension introduces a mechanism whereby the result of a query object
    may be retrieved into a buffer object instead of client memory. This allows
    the query rsult to be made available to a shader without a round-trip to
    the CPU for example by subsequently using the buffer object as a uniform
    buffer, texture buffer or other data store visible to the shader. This
    functionality may also be used to place the results of many query objects
    into a single, large buffer and then map or otherwise read back the entire
    buffer at a later point in time, avoiding a per-query object CPU-GPU
    synchronization event.

    The extension allows acquiring the result of any query object type
    supported by the GL implementation into a buffer object. The implementation
    will determine the most efficient method of copying the query result to the
    buffer.

New Procedures and Functions

    None.

New Tokens

    Accepted by the <pname> parameter of GetQueryObjectiv, GetQueryObjectuiv,
    GetQueryObjecti64v and GetQueryObjectui64v:

        QUERY_RESULT_NO_WAIT                            0x9194

    Accepted by the <target> parameter of BindBuffer, BufferData,
    BufferSubData, MapBuffer, UnmapBuffer, MapBufferRange, GetBufferSubData,
    GetBufferParameteriv, GetBufferParameteri64v, GetBufferPointerv,
    ClearBufferSubData, and the <readtarget> and <writetarget> parameters of
    CopyBufferSubData:

        QUERY_BUFFER                                    0x9192

    Accepted by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv,
    and GetDoublev:

        QUERY_BUFFER_BINDING                            0x9193

    Accepted in the <barriers> bitfield in MemoryBarrier:

        QUERY_BUFFER_BARRIER_BIT                        0x00008000

Additions to Chapter 4 of the OpenGL Core Profile Specification, Version 4.3,
"Event Model"

    Modify Section 4.2.1, "Query Object Queries"

    Add new paragraph after the fist paragraph on p. 44 ending with
    "... <id> is the name of a query object."

        Initially, zero is bound to the QUERY_BUFFER binding point, indicating
    that <params> is a pointer into client memory. However, if a non-zero
    buffer object is bound as the current query result buffer (see section
    6.1), then <params> is treated as an offset into the designated buffer
    object.

    Add new paragraph after the third paragraph on p. 44 ending with
    "... finite amount of time."

        If <pname> is QUERY_RESULT_NO_WAIT, then the query object's result
    value is returned as a single integer in <params> if the result is
    available at the time of the state query. If the result is not available
    then the destination memory location is not overwritten.

Additions to Chapter 6 of the OpenGL Core Profile Specification, Version 4.3,
"Buffer Objects"

    Modify Section 6.1, "Creating and Binding Buffer Objects"

    Add to Table 6.1: "Buffer object binding targets"

    +--------------------+--------------------------+-------------------------+
    | Target name        | Purpose                  | Described in section(s) |
    +--------------------+--------------------------+-------------------------+
    | QUERY_BUFFER       | Query result buffer      | 4.2.1                   |
    +--------------------+--------------------------+-------------------------+

Additions to Chapter 7 of the OpenGL Core Profile Specification, Version 4.3,
"Programs and Shaders"

    Modify Section 7.12, "Shader Memory Access"

    Add to the list of MemoryBarrier <barriers> bullets, p. 138

      * QUERY_BUFFER_BARRIER_BIT: Writes of buffer objects via the QUERY_BUFFER
        binding (see section 4.2.1) after the barrier will reflect data written
        by shaders prior to the barrier. Additionally, buffer object writes
        issued after the barrier will wait on the completion of all shader
        writes initiated prior to the barrier.

Additions to the AGL/GLX/WGL Specifications

    None.

GLX Protocol

    None.

Errors

  Replace second error case for QueryObject* with the following:

    An INVALID_ENUM error is generated if <pname> is not QUERY_RESULT,
    QUERY_RESULT_NO_WAIT or QUERY_RESULT_AVAILABLE.

  Add error case for QueryObject* as follows:

    An INVALID_OPERATION error is generated if the command would cause data
    to be written beyond the bounds of the buffer currently bound to the
    QUERY_BUFFER target.

New State

    Append to Table 23.73, "Miscellaneous"

    +----------------------+------+--------------+---------------+------------------------------+-------+
    | Get Value            | Type | Get Command  | Initial Value | Description                  | Sec.  |
    +----------------------+------+--------------+---------------+------------------------------+-------+
    | QUERY_BUFFER_BINDING | Z+   | GetIntegeriv | 0             | Query result buffer binding. | 4.2.1 |
    +----------------------+------+--------------+---------------+------------------------------+-------+

New Implementation Dependent State

    None.

Usage Examples

    Convenient macro definition for specifying buffer offsets:

        #define BUFFER_OFFSET(i)    ((void*)NULL + (i))

    Example 1: Using occlusion query result in shader

        // Create a buffer object for the query result
        glGenBuffers(1, &queryBuffer);
        glBindBuffer(GL_QUERY_BUFFER, queryBuffer);
        glBufferData(GL_QUERY_BUFFER, sizeof(GLuint),
                     NULL, GL_DYNAMIC_COPY);

        // Perform occlusion query
        glBeginQuery(GL_SAMPLES_PASSED, queryId)
        ...
        glEndQuery(GL_SAMPLES_PASSED);

        // Get query results to buffer object
        glBindBuffer(GL_QUERY_BUFFER, queryBuffer);
        glGetQueryObjectuiv(queryId, GL_QUERY_RESULT, BUFFER_OFFSET(0));

        // Bind query result buffer as uniform buffer
        glBindBufferBase(GL_UNIFORM_BUFFER, 0, queryBuffer);
        ...

        --- Shader ---

        ...
        uniform queryResult {
            uint samplesPassed;
        };
        ...
        void main() {
            ...
            if (samplesPassed > threshold) {
                // complex processing
                ...
            } else {
                // simplified processing
                ...
            }
            ...
        }

    Example 2: Using occlusion query result in shader only if result is available

        // Create a buffer object for the query result
        glGenBuffers(1, &queryBuffer);
        glBindBuffer(GL_QUERY_BUFFER, queryBuffer);
        glBufferData(GL_QUERY_BUFFER, 2 * sizeof(GLuint),
                     NULL, GL_DYNAMIC_COPY);

        // Perform occlusion query
        glBeginQuery(GL_SAMPLES_PASSED, queryId)
        ...
        glEndQuery(GL_SAMPLES_PASSED);

        // Get query availability and result (if available) to buffer object
        glBindBuffer(GL_QUERY_BUFFER, queryBuffer);
        glGetQueryObjectuiv(queryId, GL_QUERY_RESULT_AVAILABLE, BUFFER_OFFSET(0));
        glGetQueryObjectuiv(queryId, GL_QUERY_RESULT_NO_WAIT, BUFFER_OFFSET(4));

        // Bind query result buffer as uniform buffer
        glBindBufferBase(GL_UNIFORM_BUFFER, 0, queryBuffer);
        ...

        --- Shader ---
        
        ...
        uniform queryResult {
            uint resultAvailable;
            uint samplesPassed;
        };
        ...
        void main() {
            ...
            if (resultAvailable) {
                if (samplesPassed > threshold) {
                    // complex processing
                    ...
                } else {
                    // simplified processing
                    ...
                }
            } else {
                // default processing if no query result is available
                ...
            }
            ...
        }
        
    Example 3: Using a default value and QUERY_RESULT_NO_WAIT

        // Create a buffer object for the query result        
        // Store a default value in the buffer that will be used
        // if the query results are not available
        glGenBuffers(1, &queryBuffer);
        glBindBuffer(GL_QUERY_BUFFER, queryBuffer);
        GLuint defaultValue = 42;
        glBufferData(GL_QUERY_BUFFER, sizeof(GLuint),
                     &defaultValue, GL_DYNAMIC_COPY);
        
        // Perform occlusion query
        glBeginQuery(GL_SAMPLES_PASSED, queryId)
        ...
        glEndQuery(GL_SAMPLES_PASSED);

        // Get query results to buffer object with no wait
        // Default value remains untouched if results are not available
        glBindBuffer(GL_QUERY_BUFFER, queryBuffer);
        glGetQueryObjectuiv(queryId, GL_QUERY_RESULT_NO_WAIT, BUFFER_OFFSET(0));
        ...

    Example 4: Using transform feedback query result to fill indirect draw buffer

        // Create a buffer object for the indirect draw command        
        glGenBuffers(1, &drawIndirectBuffer);

        // Initialize draw command
        DrawArraysIndirectCommand cmd = { ... };
        glBindBuffer(GL_DRAW_INDIRECT_BUFFER, drawIndirectBuffer);
        glBufferData(GL_DRAW_INDIRECT_BUFFER, sizeof(DrawArraysIndirectCommand),
                     &cmd, GL_DYNAMIC_COPY);
        
        // Perform transform feedback query
        glBeginQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN, queryId)
        ...
        glEndQuery(GL_TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN);

        // Write query result to the primCount field of the indirect draw command
        glBindBuffer(GL_QUERY_BUFFER, drawIndirectBuffer);
        glGetQueryObjectuiv(queryId, GL_QUERY_RESULT,
                            BUFFER_OFFSET(offsetof(DrawArraysIndirectCommand, primCount)));

        // Execute the indirect draw command
        glDrawArraysIndirect(GL_TRIANGLES, BUFFER_OFFSET(0));
        ...

Issues

    1) What is QUERY_RESULT_NO_WAIT useful for?

    RESOLVED: The application may decide that it does not want to wait for the
    result of the query object if it's not available at the time. This is not
    a problem without this extension as the application can always query the
    availability using QUERY_RESULT_AVAILABLE and decide to actually get the
    results using QUERY_RESULT only if the result is available. However, when
    using query buffers, QUERY_RESULT_AVAILABLE and QUERY_RESULT alone cannot
    provide the same flexibility for shader based decision making as the
    results are either always available (if QUERY_RESULT was used) or never.
    QUERY_RESULT_NO_WAIT provides a way to query the result only if it's
    available. Combined with QUERY_RESULT_AVAILABLE, the shader can decide
    to use the result or not based on the availability (see usage example 2).

    2) Should QUERY_RESULT_NO_WAIT be accepted by GetQueryObject* in case
    there is no buffer bound to QUERY_BUFFER?

    RESOLVED: YES, for completeness.

    3) Is there any guarantee that GetQueryObject* will not wait for the
    query results to become available if <pname> is QUERY_RESULT_NO_WAIT?

    RESOLVED: There is no need to have such guarantee. An implementation may
    choose to wait for the results even in case of QUERY_RESULT_NO_WAIT,
    however, that may incur a potential performance hit in case the
    application expects it to not wait.

    4) Should the INVALID_OPERATION error be generated if a GetQueryObject*
    command would access data outside the range of the bound query buffer?

    RESOLVED: YES. This requires considering the value of <params> and the
    size of the type of the written value to determine the maximum addressed
    byte for the state query command.

    Note: This follows the precedence of the language introduced by
    ARB_pixel_buffer_object.

    5) Should we support 64 bit versions of the state query commands
    (GetQueryObjecti64v and GetQueryObjectui64v)?

    RESOLVED: YES, both for completeness and to support timer queries. In this
    case a 64 bit integer value is written to the buffer.

    6) Should the extension support all query types supported by the current
    GL implementation?

    RESOLVED: YES. There is not much value in providing additional capability
    queries that would allow the application to find out which query object
    types are supported. Also, the GL implementation can always choose to
    implement the functionality through a GPU-CPU rount-trip for query types
    that cannot be resolved to a buffer by the hardware.

    7) Is there any precedence that the pointer parameter of a glGet* command
    is treated as an offset into a bound buffer object?

    RESOLVED: YES, glGetTexImage accepts an offset into the pixel pack buffer
    in case a pixel pack buffer is bound. As pixel buffer objects are part of
    the core specification since version 2.1, no precedence is introduced by
    this extension.

    8) What should be written to the query buffer when QUERY_RESULT_NO_WAIT
    is used but the results are not available?

    DISCUSSION: Leaving the written value undefined is an option, however
    in many cases the application can benefit from having a defined value in
    the query buffer if the results are not available. The following options
    were considered to support such use cases:

        a. Write a predefined fixed value to the buffer.
        b. Write a user specified value to the buffer.
        c. Don't write anything to the buffer.

    The problem with option a. is that it may be difficult to select such
    a predefined value that would not potentially conflict with a valid
    value. Option b. could be fine, however it requires new API to specify
    this default value. Thus, option c. is considered to most preferable.

    RESOLVED: Nothing, the current value in the specified offset of the
    query buffer is left untouched.

    9) How does ARB_query_buffer_object differ from AMD_query_buffer_object?

    RESOLVED: This extension introduced a new barrier bit accepted by
    MemoryBarrier that controls the relative ordering of shader buffer writes
    and query buffer writes.

Revision History

    Rev.    Date      Author    Changes
    ----  --------    --------  ----------------------------------------------

      3   03/07/2016  drakos    Fixed typo in example #4
      2   04/02/2013  drakos    Added missing function names to the list of
                                functions accepting QUERY_BUFFER as parameter
      1   03/08/2013  drakos    Initial draft based on AMD_query_buffer_object
                                Removed suffixes
                                Updated spec language to GL 4.3
                                Added QUERY_BUFFER_BARRIER_BIT